Method filter

/**
 * Method to filter a field value.
 *
 * @param   mixed     $value  The optional value to use as the default for the field.
 * @param   string    $group  The optional dot-separated form group path on which to find the field.
 * @param   Registry  $input  An optional Registry object with the entire data set to filter
 *                            against the entire form.
 *
 * @return  mixed   The filtered value.
 *
 * @since   4.0.0
 * @throws  \UnexpectedValueException
 */
public function filter($value, $group = null, Registry $input = null)
{
    // Make sure there is a valid SimpleXMLElement.
    if (!$this->element instanceof \SimpleXMLElement) {
        throw new \UnexpectedValueException(sprintf('%s::filter `element` is not an instance of SimpleXMLElement', \get_class($this)));
    }
    // Get the field filter type.
    $filter = (string) $this->element['filter'];
    if ($filter !== '') {
        return parent::filter($value, $group, $input);
    }
    // Dirty way of ensuring required fields in subforms are submitted and filtered the way other fields are
    $subForm = $this->loadSubForm();
    // Subform field may have a default value, that is a JSON string
    if ($value && is_string($value)) {
        $value = json_decode($value, true);
        // The string is invalid json
        if (!$value) {
            return null;
        }
    }
    if ($this->multiple) {
        $return = [];
        if ($value) {
            foreach ($value as $key => $val) {
                $return[$key] = $subForm->filter($val);
            }
        }
    } else {
        $return = $subForm->filter($value);
    }
    return $return;
}